1. Technical Field
The present application relates to a technique of assessing whether a user has been able to comfortably listen to a speech sound or not. More specifically, the present application relates to a comfortableness assessment system of speech sound listening for assessing level of comfortableness in listening to speech sounds, for the “fitting” of a hearing aid or the like to provide a sound of appropriate loudness for each individual user by adjusting the amount of amplification of sounds with respect to each frequency.
2. Description of the Related Art
In recent years, people suffering from presbycusis are increasing in number due to the aging society. Even among the young, due to increased opportunities for listening to loud music for long hours as well as other influences, there is an increasing number of people suffering from hypacusia associated with acoustic traumas. Moreover, due to the downsizing and improved performance of hearing aids, users feel less of a psychological barrier against wearing hearing aids. Against this background, there is an increasing number of users who wear hearing aids on a daily basis in order to improve their conversational aural distinction abilities.
A hearing aid is a device for compensating for the deteriorated hearing of a user by increasing the amplitude of signals of specific frequencies, among various frequencies that compose sounds that are difficult for the user to hear. The amount of sound amplification which a user desires in a hearing aid varies depending on the level of deterioration in the hearing of the user. Therefore, before beginning use of a hearing aid, “fitting” is required for adjusting the amount of sound amplification in accordance with the hearing of each user.
Fitting is performed in such a manner that the output sound pressure (i.e. fluctuations in air pressure that are perceivable as a sound) of each frequency from a hearing aid is at an MCL (most comfortable level: a sound pressure level that is felt comfortable to a user). Thus, it is considered that appropriate fitting is yet to be attained under (1) an insufficient amount of amplification, or (2) an excessive amount of amplification. For example, under an insufficient amount of amplification, aural distinction of audios is not achieved, thus falling short of the purpose of wearing a hearing aid. Under an excessive amount of amplification, although distinction of audios may be possible, there is a problem in that the user may feel annoyance, which prevents them from using the hearing aid over a long time. Therefore, a fitting needs to be done in such a manner that neither (1) nor (2) occurs.
A first step of fitting is measuring an audiogram. An “audiogram” is an evaluation of a threshold value defining a smallest sound pressure of a pure tone that allows it to be heard; for example, a diagram in which, for each of a number of sounds of different frequencies, the smallest sound pressure level (decibel value) that the user can aurally comprehend is plotted against frequency (e.g., 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz).
Next, based on a fitting theory, which is a mathematical function for estimating an amount of amplification for each frequency, an amount of amplification for each frequency is determined from the resultant audiogram.
However, from an audiogram/fitting theory-based adjustment alone, one cannot know whether an optimum fitting has been realized for improving the intelligibility in aural distinction of conversations. Possible reasons are, for example: an audiogram is not in one-to-one correspondence with a conversational aural distinction ability; a person suffering from hypacusia has a narrow range of sound pressure that is felt to him or her as an appropriate loudness, which makes adjustment difficult; and so on.
Therefore, upon wearing a hearing aid which has been determined and adjusted by the aforementioned method, a hearing aid suitability test is to be carried out (see, for example, Hiroshi HOSOI et al., HOCHOUKI TEKIGOKENSA NO SHISHIN 2008, or “2008 Guidelines For Hearing Aid Suitability Test”, 2008). There are two mandatory test items in a hearing aid suitability test: (1) measurement of a speech sound intelligibility curve, and (2) measurement of an ambient noise tolerance level.
In the measurement of a speech sound intelligibility curve, both when wearing a hearing aid and when not wearing a hearing aid (naked ear), monosyllabic speech sounds are presented at 55 dB SPL (Sound pressure level), 65 dB SPL, 75 dB SPL, and 85 dB SPL; and the speech sound intelligibility at each sound pressure level is plotted for comparison. Then, if the intelligibility appears improved when wearing a hearing aid relative to when not wearing a hearing aid, it is determined as suitable.
As used herein, “speech sound intelligibility” refers to a level as to how well a monosyllabic speech sound has been aurally comprehended. A speech sound intelligibility reflects a level of aural distinction during conversations. A “monosyllabic speech sound” means either a single vowel or a combination of a consonant and a vowel (e.g., “(a)”/“(da)”/“(shi)”).
Speech sound intelligibility is assessed through the following procedure (see, for example, Kazuoki KODERA, “HOCHOKI FITTINGU NO KANGAEKATA (or “Concept of Hearing Aid Fitting”), Shindan To Chiryosha, 1999, p. 166). First, audios in the 67S list (20 speech sounds) proposed by the Japan Audiological Society are reproduced one by one, which a user is allowed to hear. Next, through oral explanation, writing, or other methods, the user is asked to answer which speech sound he or she has aurally comprehended the presented speech sound to be. Then, an evaluator matches the answers against the speech sounds which have been presented, and calculates a correctness rate, which is a rate of speech sounds that have been correctly aurally comprehended among the total of 20 speech sounds. This correctness rate is the speech sound intelligibility.
Various techniques have been disclosed in the past concerning methods of speech sound intelligibility assessment. For example, Japanese Laid-Open Patent Publication No. 9-038069 discloses a speech sound intelligibility assessment method which employs a personal computer (PC) to automatically perform correctness determination. This publication proposes a method in which monosyllabic audios are presented to a user by using a PC; the user is asked to answer with a mouse or by touching a pen to the display; the answers are received as inputs to the PC; and correctness determinations as to the presented audios and answer inputs are automatically made. Since answer inputs are received with a mouse or a pen touch, there is no need for the evaluator to distinguish and analyze the user's answers (which are given by oral explanation or writing), whereby the trouble of the evaluator is reduced.
Moreover, for example, Japanese Laid-Open Patent Publication. No. 6-114038 discloses a speech sound intelligibility assessment method in which, after audio presentation, possible choices of speech sounds are presented in the form of text characters. In this publication, choices are limited to only a small number so that the relevant speech sound can be found among the small number of characters, whereby the user's trouble of finding the character is reduced.
On the other hand, in the measurement of an ambient noise tolerance level, sounds which are read aloud are simultaneously presented with ambient noise, and after the sounds which are read aloud are heard, an assessment is made as to whether the ambient noise is tolerable or not (KODERA, et al., supra). Specifically, sounds which are read aloud are presented at 65 dB SPL, and ambient noise is presented at 55 dB SPL, and a subjective impression as to whether the ambient noise is tolerable or not is to be reported. As the subjective impression, it is to be reported whether one can endure using a hearing aid when listening to sounds which are read aloud in the presence of noise, or it is difficult to wear a hearing aid in the presence of noise. The former case is determined as suitable, whereas the latter case is determined as unsuitable.